1. Field of the Invention
This invention relates to cordless telephones, and more particularly to a cordless telephone which employs synchronization of large burst data transmission.
2. Description of the Related Art
It is quite common in the cordless telephone field to send digital information in data bursts. In traditional cordless telephone models where speech data is sent via analog transmission means, short digital data bursts are typically employed to transmit basic telephone commands from the telephone handset to the telephone base unit and vice versa. In digital cordless telephones, however, both command data and speech data are sent via short digital data bursts.
In most cordless telephones, the length of each data burst is limited in duration. Additionally, each data burst requires a relatively substantial amount of "overhead" as compared to the amount of information data actually transferred. For example, the data burst described in U.S. Pat. No. 5,073,932 (Yossifor et al) is 24 bits in length (excluding the "synchronization code" which is simply an alternating pattern of "1" and "0" used to stabilize the receiver hardware). Of these 24 bits, only five bits are used for command data, and the remaining bits are overhead.
Short data burst length and high overhead are not problematic for first generation cordless telephones due to the limited number of commands that need to be transferred between the telephone base unit and the telephone handset. However, in order to accommodate the increasing variety of services being offered by local telephone companies (e.g., Caller ID, Call-Waiting Caller ID, Caller ID Deluxe, Type 3 Telephones, etc.), it has become necessary to transmit larger streams of data from the base unit to the handset. For example, a cordless telephone user might want the ability to access Caller ID information through the handset either audibly through a speaker or visually via a text display. Moreover, these data streams often need to be transferred very quickly. Simply cascading several short data bursts in sequence will often require too much time to convey the required information due to the high overhead.
Current methods for receiving data burst transmissions are incapable of receiving long data bursts (i.e. 25 bytes or more) and/or are relatively costly to implement. The primary reason for being incapable of receiving long data bursts regards the difficulties in maintaining proper data burst synchronization between the transmitter and receiver.
In the prior art, synchronization of the transmitter and receiver is typically established only at the beginning of a data burst. Since steps are not taken during the remainder of the burst to adjust the synchronization of the signal, minor discrepancies between the transmitter and receiver clocks quickly accumulate, leading to a loss of synchronization in the form of bit-slippage (gaining or losing at least 1 bit) at the receiver.
This occurs even with cordless telephones which transmit speech data digitally. U.S. Pat. No. 5,434,905 (Maeda et al) provides an example of a digital cordless telephone which uses data bursts for both speech and data transmission. Since the synchronization is established only at the beginning of the data burst, the only way to maintain synchronization for any significant amount of time is to use crystal oscillators with extremely low tolerances in both the base unit and handset. In practice, this is a much too costly solution given the competitive nature of today's telecommunications market.
U.S. Pat. No. 5,436,937 (Brown et al) discloses a rather complex, hardware intensive method of maintaining data burst synchronization. Brown's invention is implemented using a multi-mode phase-locked loop circuit combined with an early/late bit transition accumulator. Although the multi-mode phase-locked loop circuit provides a way of maintaining synchronization between the handset and base unit throughout a data burst, it is too costly for use in cordless telephones.